Wireless multiple-input-multiple-output (MIMO) systems represent an advance in wireless communication. MIMO systems employ two or more antennas at the transmitting and/or receiving ends of a wireless link. The multiple antennas improve data transmission rates, while holding radio bandwidth and power constant.
A MIMO transmitter transmits an outgoing signal using multiple antennas by demultiplexing the outgoing signal into multiple sub-signals and transmitting the sub-signals from separate antennas. MIMO exploits the multiple signal propagation paths to increase throughput and reduce bit error rates. Using MIMO techniques the rate of transmission increases linearly depending on the local environment.
In a conventional wireless MIMO system, when base stations in adjacent or closely located cells transmit concurrently or simultaneously, signals transmitted from a first base station in a first cell may interfere with signals transmitted from a second base station in an adjacent or closely located second cell. In more detail, signals transmitted by the first base station in the first cell are seen as interference by mobiles located in the adjacent second cell. Similarly, signals transmitted by the second base station are seen as interference by mobiles in the first cell. This type of interference is known as inter-cell interference. Inter-cell interference prevents faster data transmission between base station and mobile within a given cell (known as intra-cell transmission).
Conventionally, inter-cell interference is reduced using frequency reuse methods. In this approach, different frequency bands are used within adjacent cells because adjacent cells create the most significant interference to each other. But, conventional frequency reuse methods reduce intra-cell data transmission rates between the base station and mobiles in the same cell.
For example, in a frequency reuse of 3 method, each base station transmits data using a bandwidth of F/3, where F is the full bandwidth for the air interface. In this example, each base station's data transmission rate is only about one-third of the nominal data transmission rate. As one can appreciate, this is significantly less than if inter-cell interference were not present. Such a decrease in bandwidth within a cell is undesirable, but unavoidable in the conventional art.